1. Field of the Invention
The present invention relates to a switching power supply, wherein electric power is controlled by turning a switching element on and off, and to a distributed parameter structure for use in said switching power supply.
2. Description of the Prior Art
FIG. 1 shows a schematic block diagram of a fly-back converter, which is an example of prior art switching power supplies.
In FIG. 1, an input power supply Vin is connected to an X capacitor C6, to a line filter T2 which is a filter inductor, to a diode bridge D3, and to a bulk capacitor C5.
A voltage Vb of the bulk capacitor C5 is connected to one end of a primary winding N1 of a transformer T1, the other end of the primary winding N1 of the transformer T1 being connected to a voltage Vds of the drain of a switching element Q1, and the source of the switching element Q1 is connected to a stable potential COM.
A capacitor C1, a resistance R1, and a diode D1 configure a CRD snubber circuit 10 which is a primary snubber circuit. The anode of the diode D1 is connected to the drain Vds of the switching element Q1, one end of the resistance R1 and one end of the capacitor C1 being connected to the cathode of the capacitor D1, while the other end of the resistance R1 and the other end of the capacitor C1 being connected to the voltage Vb.
The CRD snubber circuit 10 branches off at the voltage Vds in the main line, which is a route from the primary winding N1 of the transformer T1 to the drain of the switching element Q1.
A secondary winding N2 of the transformer T1 is connected to an output Vout via a rectifier circuit of a diode D2 and a smoothing circuit of a capacitor 4.
A capacitor C2 and a resistance R2 are connected serially to configure a CR snubber circuit which is a secondary snubber circuit, and are connected in parallel to the diode D2.
Behaviors of the above-mentioned prior art embodiment in FIG. 1 will be explained hereunder. The input power supply Vin is rectified at the diode bridge D3, smoothed at the bulk capacitor C5, and becomes the voltage Vb. The switching element Q1 turns on and off the voltage Vb that is applied to the primary winding N1 of the transformer T1. A voltage induced at the secondary winding N2 of the transformer T1 is rectified at the diode D2, smoothed at the capacitor C4, and becomes the output voltage Vout.
The voltage Vds changes in square waves when the switching element Q1 is turned on and off. Also, a surge is generated when the switching element Q1 is turned on and off. The surge is influenced by the parasitic inductance and parasitic capacity of the transformer T1 and by the output capacity and switching characteristics of the switching element Q1. The CRD snubber circuit 10 suppresses a voltage surge, which is generated when the switching element Q1 is turned off.
More specifically, when the switching element Q1 is turned on, the voltage Vds is low and the diode D1 is turned off. Then, when the switching element Q1 is turned off, the voltage Vds increases while a voltage surge is generated. When the voltage Vds increases, the diode D1 is turned on while the capacitor C1 provides an electric charge. Increase of the voltage Vds is suppressed when the capacitor C1 provides an electric charge. Electric charges of the capacitor C1 are discharged at the resistance R1.
A part of the noise that is generated when the switching element Q1 is turned on or off is passed on to the input power supply Vin via the diode bridge D3, the line filter T2, and the X capacitor C6. The main inductance of the line filter T2 attenuates common mode elements of noise. The leaked inductance of the line filter T2 and the X capacitor C6 attenuate normal mode elements of noise.
FIG. 2 illustrates the waveforms of the voltage Vds that was generated when the switching element Q1 was turned off in the prior art embodiment of FIG. 1. A voltage surge was generated when the frequency was approximately 7 MHz. When the diode D1 was turned on at the high voltage point P, the capacitor C1 clamped the oscillations of the voltage surge of the voltage Vds. The amplitude of the voltage surge of the voltage Vds was attenuated gradually as the energy became heat, noise, and others.
FIG. 3 illustrates conduction noise characteristics of the prior art embodiment in FIG. 1. In FIG. 3, the portion A shows the noise which peaked at the frequency of 8 MHz. The portion A was generated when a voltage surge of the voltage Vds in FIG. 2 became conduction noise. The reason why frequencies did not match in FIGS. 2 and 3 was that they were mainly influenced by parasitic capacities of probes when waveforms were measured.
Also, the CR snubber circuit comprising the capacitor C2 and the resistance R2 suppresses a voltage surge generated at the diode D2.
Moreover, some of the prior art switching power supplies have wirings equipped at their transformers in order to eliminate common mode signals (for example, see the Japanese Utility Model Gazette 1988-30230 according to the concept proposed by the present applicant).
An object of such prior art embodiments is to realize an insulated DC power supply circuit that is less influenced by common mode signals by means of windings of a transformer. The object, however, cannot be a cause or a motivation of suppression of surges generated at a switching element. Furthermore, the object does not include any intention to add windings to filter inductors.
On the other hand, FIG. 4(a) and FIG. 4(b) are schematic diagrams of a prior art micro strip line and show a distributed parameter structure.
FIG. 4(a) shows a perspective diagram. A distributed parameter line Z1 branches off at the point S in the main line which runs from the input port to the output port. The distributed parameter line Z1 is open-ended and becomes an open stub.
When the line length L of the distributed parameter line Z1 is       1    4    ·  λ(λ is a wavelength), the distributed parameter line Z1 acts as a filter for the wavelength λ and separates specific frequency elements of signals propagated in the main line.
FIG. 4(b) shows a cross section. The distributed parameter line Z1 is formed by a conductor of the width W and the thickness t on a flat plate. A stable potential surface Z2 is connected to a stable potential GND, is formed by a conductor on a flat plate which is sufficiently wider than the distributed parameter line Z1, and is arranged in parallel with the distributed parameter line Z1. A dielectric Z3 having the thickness h and the relative dielectric constant ∈r is formed so that it is placed between the distributed parameter line Z1 and the stable potential surface Z2.
Accordingly, in the prior art distributed parameter structure, the distributed parameter line Z1 is formed as a linear and flat conductor on a flat surface. The stable potential surface Z2 is formed as a flat conductor.
Next, the distributed parameter line Z1 is explained in detail. A frequency f and a wavelength λ have approximately the following relationship:λ=C/f/Sqrt (∈r)
Here, C is the speed of light (3*108 m/s) and ∈r is the relative dielectric constant of the dielectric Z3 (4.21 in the case of polyurethane). A wavelength in the dielectric Z3 is proportional to the inverse number of the square root of the relative dielectric constant ∈r. That is, the wavelength is reduced to 1/Sqrt (∈r) in comparison with the wavelength in vacuum.
For example, when f=7 MHz and ∈r=4.21 are given, λ=20.9 m is produced and consequently             1      4        ·    λ    =      5.22    ⁢                   ⁢    m  is obtained. The characteristics of the distributed parameter line Z1 are almost determined by its line length L. Influences of its width W and thickness t, the thickness h of the dielectric Z3, and others are small.
However, these switching power supplies have problems such as increased losses due to a resistance R1 and deterioration of conduction noise characteristics.
In addition, if a prior art distributed parameter structure Z1 is applied in a wavelength corresponding to a frequency band (MHz band) that is liable to cause a problem for switching power supplies, there is another problem of larger size.
More specifically, if a distributed parameter line Z1 of 5.22 m is formed linearly, a switching power supply becomes larger in size and consequently impractical.